The Sermon on the Interloper is the collective force event that led to The Battle of Tull. It was The Man in Black's trap for Roland Deschain in Tull. The Man in black coherced Sylvia Pittston that Roland was the Interloper, and that she should turn the town against him.
Sylvia believed The Man in Black, and when preaching abot the Interloper- the one who makes disaster and sin- Roland waited in the shadows and knew she was talking about him.